This invention relates to the art of geophysical prospecting in general and to the art of radioactivity well logging in particular. Specifically, the invention relates to logging wherein radiation scattered by fluids passing into and along a borehole is analyzed to provide an indication of the location of fluid entry into the borehole and the composition of the fluid.
In preparation for producing fluids from a borehole such as a cased oil wellbore, it is customary to perforate the casing at various locations along the length of the casing that correspond to projected regions of oil flow. However, these oil flow projections are not always accurate. Occasionally, regions expected to produce substantial oil flow into the borehole produce little or no oil flow at all, or these regions produce fluid flows of widely disparate oil and water content throughout the length of the borehole. Because the fluid produced at the head of the well (production fluid) is a composite of all of the fluids flowing into the borehole through the perforations in the wellbore casing, it would be desirable to identify regions along the length of the wellbore casing that are responsible for the introduction into the borehole of fluids having a low oil content or high water content. Following the identification of such low oil yield regions, appropriate countermeasures such as partial re-casing or re-cementing of the wellbore may be undertaken to improve the quality of the production fluid.
Several methods of water flow monitoring are available for locating various types of fluid movement. One method is discussed in an article by R. McKinley, F. Bower, and R. Rumble, entitled "The Structure and Interpretation of Noise From Flow Behind Cemented Casing", paper SPE 3999, presented at the SPEAIME 47th Annual Fall Meeting (Oct. 8-11, 1972). The article discloses an acoustic logging technique wherein acoustical waves generated by turbulence from high energy fluids is measured to provide an indication of the location of certain types of fluid movement.
Several attempts have been made recently to apply the oxygen activation technique to obtain a volume flow rate of the water phase of the fluid produced by the well using the TMD (Thermal Multigate Decay) pulsed logging system. The oxygen activation technique involves irradiating a borehole environment with neutrons sufficiently energized to induce the transformation of oxygen to an unstable isotope of nitrogen in accordance with the following reaction: EQU O.sup.16 (n, p)N.sup.16 ; t.sub.1/2 =7.36 sec.
where t.sub.1/2 represents the half-life of the artificially produced isotope. Due to the inherent instability of the nitrogen isotope, the isotope spontaneously decays back to oxygen while simultaneously emitting gamma rays of characteristic energies. Gamma radiation caused by the decay of the nitrogen isotope is detected by a gamma radiation detector and a representative signal is produced based on this detected gamma radiation. A measure of the oxygen content of the fluid in the vicinity of the detector may be derived from this signal, which measure may be correlated with the amount of water in the fluid.
In an article written by G. Lamb and G. Webber, entitled "Measurement of Water Flow in Deviated Production Wells by Oxygen Activation Logging", SPWLA 24th Annual Logging Symposium, June 27-30, 1983 (paper Z), oxygen activation measurements are conducted without compensating for the significant contribution resulting from the neutron activation of sodium (Na.sup.23 (n, .alpha.)F.sup.20 ; t.sub.1/2 =10.7 sec.) and chlorine (Cl.sup.37 (n, .alpha.)P.sup.34 ; t.sub.1/2 =12.4 sec.). Because the contributions from sodium and chlorine to the total observed activation are appreciable, failure to take these contributions into account may result in a distorted indication of the composition and flow of water into the borehole along the measured borehole section or interval.
U.S. Pat. No. 4,287,415 to D. M. Arnold, a coinventor herein, pertains to a method for measuring the salinity of water flowing either within or behind the casing of a borehole wherein 14 MeV neutrons are used to irradiate the flowing water in order to induce activation of certain components of the water. Gamma radiation induced from the O.sup.16 (n, p)N.sup.16 reaction and either the Na.sup.23 (n, .alpha.)F.sup.20 or the Cl.sup.37 (n, .alpha.)P.sup.34 reaction is measured to provide an indication of the salinity of the flowing water. However, this method uses an energy pulse height analyzer to separate sodium, chlorine, and oxygen activation components.
It is desirable to obtain information concerning certain characteristics of the water entering the borehole casing through various sections or intervals along the length of the borehole. Such characteristics include the volume flow rate of the water phase of the production fluid and the salinity of the water phase of the production fluid.
Accordingly, it is an object of the present invention to provide a well logging method which provides an indication of the salinity and volume flow rate of the water phase of the production fluid of a borehole.
Another object of the present invention is to provide a well logging method for the measure of the salinity and volume flow rate of the water phase of the production fluid of a borehole that may be obtained through the use of a conventional gamma ray detection system without the need for special modifications to the system.
Still another object of the present invention is to provide a well logging method that provides an indication of the salinity and volume flow rate of the water phase of the production fluid of a borehole that does not require the use of gamma ray spectroscopy to provide an indication of water salinity.
These and other objects and advantages of the present invention will become apparent from the detailed description that follows.